1. Field of the Invention
This invention relates to composite sheet material and more particularly, to a method of preparing a composite sheet material from a polyurethane polymer and porous sheet material.
2. Description of the Prior Art
Natural leather, appropriately finished, is valued for its durability and aesthetic characteristics in a plurality of uses. Due to the scarcity of leather and the increased cost of processing leather for particular applications, economics have dictated that synthetic materials be substituted in certain applications where leather goods had been used. Such synthetic materials have been proposed and used in the areas of shoe uppers, upholstery, clothing, luggage making, book binding and similar applications. Because these various applications require varying physical, chemical and aesthetic qualities, different processes using different materials must be used to obtain an acceptable product which is comparable to natural leather; although in most instances these synthetics are readily distinguishable from natural leather.
One method of preparing a synthetic as a substitute for leather involves impregnating and/or coating a porous material, for example, cloth, with a polyurethane, vinyl or a similar material. Polyurethanes have met with wide acceptance as a coating or impregnating composition due to their capability of wide variation in chemical and physical properties, particularly their flexibility and chemical resistance.
Objectives in preparing these synthetic substitutes for leather are that they provide: (1) sheets especially suitable for leather-like and upholstery uses; (2) sheets of uniform width as commonly used in the textile industry (unlike natural products which sustain substantial weight and area losses in cutting and finishing); (3) end use versatility, for example, under a variety of exposure conditions where certain chemical treatments will assist maintenance and useful lifetime of properties; (4) and most importantly, a product with the strength, hand, drape and softness comparable to natural leather.
Further, an impregnated fabric sheet material when used for shoe uppers, should be characterized by a leather-like appearance, with no undesirable fabric show-through, good water vapor permeation into the uncoated inside of the upper, and leather-like grain break (minimal gross wrinkling). "Leather-like grain break", as recognized in the leather and upholstery industries, is manifested in the behavior of well finished leather when folded or crumpled. The leather fold is characterized by a smooth, curved contour, frequently with numerous fine wrinkles in the compressed region of the fold area. This is contrasted with sharp creases or gross wrinkles formed when papers or films are folded; this kind of undesirable appearance is known as "pin wrinkling".
In another application it is desired to provide a strengthened fabric that can be sanded or buffed giving an aesthetically pleasing surface and then used without additional coating as an upholstery material. In upholstery, important characteristics include strength, reduced bias elongation, and enhanced appearance, i.e. a well filled fabric, without substrate show-through, which has softness, drape and conformability. The bias elongation is important in shoe uppers as well as upholstery and reflects a desirable resistance to stretch in the more highly stressed areas of the fabric in the end use; excessive stretch in a fabric will result in pin wrinkling and show-through.
In another application it is desired to provide coated fabrics with increased strength and enhanced appearance. These coated fabrics show actual textile patterns while having improved chemical and physical properties.
Polyurethane polymers as coatings or impregnants for fabric have long been recognized as providing some of the aforementioned characteristics. For example, polyurethanes can be made which are highly resistant to solvents and abrasion, conferring dry-cleanability and outstanding durability to coated fabrics. The basic chemistry of polyurethanes, involving reactions between the isocyanate groups and molecules with multiple reactive hydrogens, such as polyols and polyamines, afford great versatility and variability in final chemical and physical properties by the selection of intermediates to achieve processability and the desired balance of end use performance requirements.
There are various methods for applying polyurethane solutions or other post curable liquid polymers to porous substrates which are well known to those skilled in the art. An article in Journal of Coated Fabrics, Vol. 7 (July 1977), pp. 43-57 describes some of the commercial coating systems, e.g. reverse roll coating, pan fed coater, gravure and the like. Brushing and spraying may also be used to coat polyurethanes on porous substrates. These polyurethane solutions, after impregnation or coating on the porous substrate, are dried or cured by a method such as heated air, infrared radiation and the like. Characteristic of these processes is the deposition of a polymer and a film-like layer which tends to produce a coated fabric which folds in undesirable sharp creases rather than a leather-like grain break.
Another method of combining polyurethane solutions with porous substrates is disclosed in U.S. Pat. No. 3,208,875. Briefly, this method involves the application of a solution of a polymer in an organic solvent to a substrate (such as a needle punched polyester batt) with subsequent bathing of the polymeric layer with a mixture of an organic solvent for the polymer and a non-solvent for the polymer, that is at least partially miscible with the solvent until the layer is coagulated into a cellular structure of innerconnected micropores. The solvent is removed from the coating layer along with the non-solvent to produce a solvent-free microporous layer. Although this process yields acceptable properties for a polyurethane impregnated fabric, it has the disadvantages of an organic solvent system particularly when high performance polyurethanes are utilized which require relatively toxic and high boiling solvents.
Polyurethane dispersions in organic vehicles have been proposed and used to coat fabrics. U.S. Pat. No. 3,100,721 discloses dispersions made by the addition of non-solvent to polyurethane solutions. A dispersion, applied to a substrate, is coagulated by further addition of a non-solvent. Although this approach has been used with some success, it involves two major limitations: (1) the vehicle of the dispersion is substantially organic, since relatively small amounts of non-solvent, preferably water, are needed to form a dispersion; and (2) there is a narrow useful range of added non-solvent, so that reproducible results are difficult to obtain.
Although useful products based on solutions or dispersions of polyurethanes in organic vehicles have been provided, a process using water dilutable polyurethane compositions is highly desirable to overcome the impediments of the prior art processes.
Thus, in accordance with the present invention, a process for the preparation of polyurethane polymer impregnated porous substrates is provided in which water is the vehicle for the polyurethane polymer composition.
Further, in accordance with the present invention, the above disadvantages described with the prior art systems, are overcome to provide a leather-like product for a plurality of uses by a single process.
Further, in accordance with the present invention, a process is provided to produce coated fabric.
Further, the attendant advantages of an aqueous system are provided by the present invention.